Wireless communication system

ABSTRACT

In order to transmit a large amount of data in series at a time with a MIMO communication scheme while avoiding deterioration of decoding characteristics due to change over time by a channel matrix, a wireless communication system uses an open-loop type and a closed-loop type of MIMO communication modes in combination and switches to the open-loop MIMO communication mode in response to the information that the amount of data to be transmitted at a time has exceeded a predetermined amount of bits or a predetermined transmission time during data transmission under the closed-loop MIMO communication mode. By discontinuing useless closed-loop MIMO communication and switching to the open-loop MIMO communication mode that is better than Zero-forcing, the decoding characteristics are prevented from simply becoming deteriorated.

This application is a continuation of U.S. Reissue application Ser. No.13/901,182, filed May 23, 2013, which is a continuation of U.S. Reissueapplication Ser. No. 13/326,024, filed Dec. 14, 2011 (now U.S. Pat. No.RE44,320); each of the foregoing applications is also an application forthe reissue of U.S. Pat. No. 7,634,017, which corresponds to U.S. patentapplication Ser. No. 11/108,664, filed Apr. 19, 2005, thus the presentapplication is a continuation reissue application.

CROSS REFERENCES TO RELATED APPLICATIONS

The present invention contains subject matter related to Japanese PatentApplication JP 2004-134744 filed in the Japanese Patent Office on Apr.28, 2004, the entire contents of which being incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wireless communication system such asa wireless LAN (Local Area Network) for communicating between aplurality of wireless stations, and in particular, to a wirelesscommunication system for realizing broadband wireless transmission incommunication environments such as in homes.

Further more specifically, the present invention relates to a wirelesscommunication system for expanding transmission capacity bycommunication in which spatial multiplexing is utilized (MIMOcommunication), pairing a transmitter having a plurality of antennaswith a receiver having a plurality of antennas. In particular, thepresent invention relates to a wireless communication system fortransmitting a large amount of data in series at a time with a MIMOcommunication scheme, avoiding deterioration of decodingcharacteristics.

2. Description of the Related Art

Canonical standards concerning wireless networks can include IEEE (TheInstitute of Electrical and Electronics Engineers) 802.11 (e.g., seenon-patent document 1), HiperLAN/2 (e.g., see non-patent document 2 or3), IEEE802.15.3, and Bluetooth communication, for example. IEEE802.11has enhanced standards such as IEEE802.11a (e.g., see non-patentdocument 4), b, and g depending on differences of wireless communicationschemes and frequency bands.

IEEE802.11a supports a modulation scheme for achieving a maximumcommunication speed of 54 Mbps. However, there is desired a standard forrealizing a higher bit rate for the communication speed. For example, inIEEE802.11n, with the aim of establishing a wireless LAN technology forrealizing a high speed exceeding an actual throughput of 100 Mbps, thenext generation of wireless LAN standards is being developed.

As a technology for realizing a higher speed of wireless communication,MIMO (Multi-Input Multi-Output) communication is coming to attention.This is a technology for expanding transmission capacity and achievingimprovement in communication speed by realizing a spatial multiplexingtransmission channel (hereinafter also referred to as “MIMO channel”)with a plurality of respective antenna elements at a transmitter and areceiver. In the MIMO communication, good frequency utilizationefficiency is obtained due to utilization of spatial multiplexing.

In a MIMO communication scheme, the transmitter distributes transmissiondata to a plurality of antennas and transmits it through a plurality ofvirtual MIMO channels, and the receiver obtains reception data byprocessing signals received by a plurality of antennas. The MIMOcommunication scheme utilizes channel characteristics as described anddiffers from a mere transmission/reception adaptive array. That is, thetransmitter performs space-time coding on a plurality of transmissionsignals, which are then multiplexed, distributed to M antennas, andtransmitted to a plurality of MIMO channels. The receiver performsspace-time decoding on reception signals received by N antennas via thechannels to obtain reception data. In this case, a channel model iscomposed of a radio wave environment around the transmitter (transferfunction), a structure of channel space (transfer function), and a radiowave environment around the receiver (transfer function). In the case ofmultiplexing signals transmitted from each antenna, there occurscrosstalk. However, by signal processing at the receiver, eachmultiplexed signal can be extracted correctly without crosstalk.

As MIMO transmission schemes, a variety of schemes are proposed;however, how to exchange channel information between the transmitter andthe receiver in accordance with an antenna configuration is a big issuefor implementation.

In the case of exchanging the channel information, it is easy to performa method of transmitting known information (preamble information) onlyfrom the transmitter to the receiver. In this case, the transmitter andthe receiver are independent of each other to perform spatialmultiplexing transmission, and this is called an open-loop type of MIMOtransmission. As an extension of this method, there is a closed-looptype of MIMO transmission for producing an ideal spatial orthogonalchannel between the transmitter and the receiver by feedback of preambleinformation also from the receiver to the transmitter.

The open-loop type of MIMO transmission can include V-BLAST (VerticalBell Laboratories Layered Space Time) scheme for example (e.g., seepatent document 1). The transmitter does not provide an antennaweighting factor matrix, and simply multiplexes and transmits signalsfor each antenna. In other words, a feedback procedure for obtaining theantenna weighting factor matrix is entirely omitted. The transmitterinserts training signals for performing channel estimation at thereceiver, in a time-division manner, for example for each antenna,before transmitting multiplexed signals. On the other hand, the receiverperforms the channel estimation using the training signals at a channelestimation unit and calculates a channel information matrix Hcorresponding to each antenna pair. By combing zero-forcing andcanceling skillfully, a signal-to-noise ratio by utilizing a degree offreedom of each antenna that is caused by the canceling is improved anda degree of certainty of decoding is enhanced.

Further, as an ideal form for the closed-loop type of MIMO transmission,there is known a SVD-MIMO scheme utilizing singular value decomposition(SVD) of a propagation path function (e.g., see non-patent document 5).

In the SVD-MIMO transmission, UDV^(H) is obtained by performing thesingular value decomposition of a numerical matrix whose elements denotechannel information corresponding to each antenna pair, namely thechannel information matrix H, and the transmission antenna weightingfactor matrix V and the reception antenna weighting factor matrix U^(H)are obtained. Thus, each MIMO channel is expressed as the diagonalmatrix D having the diagonal elements that are the square root of eacheigenvalue λ_(i), and signals can be multiplexed to be transmittedwithout any crosstalk. In this case, there can be realized a pluralityof logically independent, space division (i.e., spatial orthogonalmultiplexing) transmission channels.

It is generally assumed that, based on transmission channel information,the transmitter calculates an optimum antenna weighting factor andoptimizes a coding rate and a modulation scheme that are applied to bitstreams for each transmission antenna in the closed-loop type of MIMOscheme and thereby more appropriate information transmission can berealized. However, there is a problem that the closed-loop type of MIMOcommunication, in order to be introduced as a real system, needs higherfrequency of feedback from the receiver to the transmitter in the caseof a large channel fluctuation due to a move of thetransmitter/receiver.

The open-loop type and the closed-loop type of MIMO communicationschemes will be described below.

FIG. 7 schematically shows the configuration of a MIMOtransmitter/receiver of the open-loop type. In this case, thetransmitter transmits a data frame containing a preamble signal(TxPreamble) for estimating a propagation path. The receiver obtains achannel matrix H based on the received preamble signal and performsweighted reception of data division using a reception antenna weightcalculated based on the channel matrix H. A feedback procedure is notperformed from the receiver to the transmitter.

FIG. 8 schematically shows the configuration of a MIMOtransmitter/receiver of the closed-loop type. In this case, thetransmitter transmits a preamble signal (TxPreamble1) for estimating apropagation path. The receiver can obtain a channel matrix H based onthe received preamble signal and sends feedback of a preamble signal(RxPreamble) for estimating the propagation path. The transmitter canobtain the channel matrix H based on the received preamble signal.Further, the transmitter obtains a transmission weight matrix based onthe obtained channel matrix H and performs weighted transmission of adata frame to which a preamble signal (TxPreamble2) is attached. Thereceiver can obtain a new channel matrix based on the preamble signal(TxPreamble2) and performs weighted reception of a data frame using areception weight matrix calculated from the channel matrix.

It is possible to exchange the preamble signals (TxPreamble andRxPreamble) between the transmitter and the receiver along with anRTS/CTS sequence for solving a hidden terminal. Further, the transmittermay transmit the preamble signal just one time. Furthermore, in the casewhere the reversibility of the propagation path is valid, thetransmission may be made in order of RxPreamble to TxPreamble.

Thus, in both transmission schemes of the open-loop type and theclosed-loop type, the receiver employs preamble information thatestimates channel information of a certain time and demodulates theremaining data division.

In a communication system (such as the MIMO transmission scheme) forperforming weighted transmission/reception based on the channel matrix Hobtained from propagation path conditions, change over time by thechannel matrix H becomes a problem. The channel matrix H changes everymoment because of a change in, for example, room temperature or otheratmospheres, a change in a reflected path due to a move of a person or adevice, or the like. In particular, in the case of transmitting a largeamount of data at a time, there occurs a channel fluctuation over timeand the accuracy of the preamble information becomes deteriorated, sothat the problem comes to the surface.

For example, in the processes of data transmission in the closed-looptype of MIMO communication system shown in FIG. 8, if transmission datais long, it is realistic that the transmitter inserts TxPreambles atfixed periods as necessary (see FIG. 9) and the receiver acquireschannel matrices successively to establish communication. At this time,in the case of the closed-loop type, since the transmitter hasestablished a transmission antenna weight, it is needless to say thatthe accuracy deterioration of the preamble information largely affectsdecoding characteristics.

[Patent document 1] Japanese Unexamined Patent Publication No. Hei10-84324

[Non-patent document 1] International Standard ISO/IEC 8802-11:1999 (E)ANSI/IEEE Std 802.11, 1999 Edition, Part11: Wireless LAN Medium AccessControl (MAC) and Physical Layer (PHY) Specifications

[Non-patent document 2] ETSI Standard ETSI TS 101761-1 V1.3.1 BroadbandRadio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC)Layer; Part1: Basic Data Transport Functions

[Non-patent document 3] ETSI TS 101 761-2 V1.3.1 Broadband Radio AccessNetworks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part2:Radio Link Control (RLC) sublayer

[Non-patent document 4] Supplement to IEEE Standard for Informationtechnology-Telecommunications and information exchange betweensystems-Local and metropolitan area networks-Specific requirements-Part11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY)specifications: High-speed Physical Layer in the 5 GHZ Band

[Non-patent document 5] http://radio3.ee.uec.ac.jp/MIMO(IEICE_TS).pdf(as of Oct. 24, 2003)

SUMMARY OF THE INVENTION

It is desirable to provide a wireless communication system capable ofexpanding transmission capacity by communication in which spatialmultiplexing is utilized (MIMO communication), pairing a transmitterhaving a plurality of antennas with a receiver having a plurality ofantennas.

It is also desirable to provide a wireless communication system capableof transmitting a large amount of data in series at a time with a MIMOcommunication scheme, avoiding deterioration of decoding characteristicsdue to change over time by a channel matrix.

According to a first embodiment of the present invention, there isprovided a wireless communication system in which a transmitter having aplurality of antennas and a receiver having a plurality of antennas forma pair and spatially multiplex and communicate a signal. The wirelesscommunication system includes a closed-loop spatial multiplexingcommunication mode in which the transmitter transmits a signal foracquiring channel information to the receiver and the receiver sendsfeedback of the channel information to the transmitter, an open-loopspatial multiplexing communication mode in which only the transmittertransmits a signal for acquiring channel information to the receiver,and a mode switcher configured to switch between the closed-loop spatialmultiplexing communication mode and the open-loop spatial multiplexingcommunication mode.

In this context, the term “system” signifies a logical set of aplurality of apparatuses (or function modules to implement a specificfunction). It is indefinite about whether or not the apparatuses orfunction modules are contained in a single cabinet.

In a wireless communication system according to an embodiment of thepresent invention, a transmitter having a plurality of antennas and areceiver having a plurality of antennas form a pair and spatiallymultiplex signals to perform MIMO communication.

In general, a MIMO communication system is broadly divided into anopen-loop type for transmitting known information (preamble information)only from the transmitter to the receiver and a closed-loop type forproducing an ideal spatial orthogonal channel between the transmitterand the receiver by feedback of the preamble information also from thereceiver to the transmitter.

The closed-loop type of MIMO scheme can realize more appropriateinformation transmission. However, there is a problem that theclosed-loop type of MIMO scheme needs higher frequency of feedback fromthe receiver to the transmitter in the case of a large channelfluctuation due to a move of the transmitter/receiver.

For example, in the case of transmitting a large amount of data at atime, there occurs a channel fluctuation over time and the accuracy ofthe preamble information becomes deteriorated, so that the problem comesto the surface. That is, due to a channel fluctuation during successivedata communication, a channel matrix changes, and a transmission weightmatrix V changes accordingly. Assume that the transmitter continues touse an old transmission weight matrix V by the closed-loop type of MIMOcommunication. As a result, although there should be originally formed alogically independent MIMO channel of which spatial orthogonality ismaintained, crosstalk occurs between MIMO channels due to a differencebetween a transmission weight matrix V to be used and a correcttransmission weight matrix V_(new), so that decoding characteristicsbecome extremely deteriorated.

According to SVD-MIMO communication which typically represents theclosed-loop type of MIMO scheme, there can be originally formed an idealspatial orthogonal channel between the transmitter and the receiver;however, as a result of having broken off learning of the channel matrixduring long data communication, the decoding characteristics of thereceiver in the closed-loop type of MIMO communication becomedeteriorated to the same degree as those of Zero-forcing (ZF) which hasespecially low decoding characteristics among MIMO schemes of theopen-loop type. This is partly because both weighting schemes aresimilar in that a reception signal is multiplied by a channel inversematrix H⁻¹ as a reception weight in Zero-forcing or multiplied by U^(H)as a reception weight in SVD-MIMO.

The inventors of the present invention consider that it is useless tocontinue the closed-loop type of MIMO communication in a state of thedeteriorated decoding characteristics and performing receptionprocessing by the open-loop type of MIMO communication other thanZero-forcing brings about better decoding characteristics.

A wireless communication system according to an embodiment of thepresent invention uses the open-loop type and the closed-loop type ofMIMO communication schemes in combination, and a mode switcher switchesseamlessly between the closed-loop type of spatial multiplexingcommunication mode and the open-loop type of spatial multiplexingcommunication mode in accordance with a change in a propagation path, sothat the deterioration of the decoding characteristics can be restrainedwithin a certain limit.

For example, the mode switcher switches to the open-loop spatialmultiplexing communication mode in response to the information that theamount of data to be transmitted at a time has exceeded a predeterminedamount of bits or a predetermined transmission time during datatransmission under the closed-loop spatial multiplexing communicationmode. Thus, by discontinuing useless closed-loop MIMO communication andswitching to the open-loop MIMO communication mode that is better thanZero-forcing, the decoding characteristics are prevented from simplybecoming deteriorated.

Alternatively, the mode switcher may switch to the open-loop spatialmultiplexing communication mode, estimating that the channel matrix haschanged over time, in response to the information that an error rate hasexceeded a predetermined value during data transmission under theclosed-loop spatial multiplexing communication mode.

Further, in the case of adopting the SVD-MIMO communication as theclosed-loop MIMO communication, the receiver can acquire the channelmatrix H by channel estimation even during data reception and also acorrect transmission weight matrix V_(new) by singular valuedecomposition. The receiver may compare the correct transmission weightmatrix V_(new) with an old V that the transmitter continues to use,estimate that the channel matrix has changed over time when thedifference between the correct transmission weight matrix V_(new) andthe old V exceeds a predetermined value, and switch to the open-loopspatial multiplexing communication mode.

Further, the transmitter transmits data that the transmitter does notapply a transmission weight V to under the open-loop spatialmultiplexing communication mode, and thereby the deterioration ofdecoding characteristics of the receiver can be restrained.

Furthermore, the receiver employs an open-loop spatial multiplexingcommunication scheme (such as MMSE, BLAST, MMSE+BLAST, and MaximumLikelihood Estimation) other than a communication scheme (such asZero-forcing) for providing a reception weight based on an inversematrix of a channel matrix obtained from propagation path conditionsunder the open-loop spatial multiplexing communication mode, so thatbetter decoding characteristics can be obtained compared to the casewhere only the closed-loop spatial multiplexing communication continuesto be used, even if propagation path conditions have changed.

In a wireless communication system according to an embodiment of thepresent invention, the transmitter can switch to the open-loop spatialmultiplexing communication mode, estimating that the channel matrix haschanged over time, in response to the information that the informationthat the amount of data to be transmitted at a time has exceeded apredetermined amount of bits or a predetermined transmission time duringdata transmission under the closed-loop spatial multiplexingcommunication mode. In this case, the transmitter transmits data thatthe transmitter does not apply a transmission weight to and also informsthe receiver that the transmitter has switched to the open-loop spatialmultiplexing communication mode. Alternatively, in response to theinformation from the receiver that the receiver has switched to theopen-loop spatial multiplexing communication mode, the transmitter alsomay switch to the open-loop spatial multiplexing communication mode.

Furthermore, the receiver can switch to the open-loop spatialmultiplexing communication mode, estimating that the channel matrix haschanged over time, according to a change in the transmission weight Vobtained successively by singular value decomposition of the channelmatrix and deterioration of reception characteristics such as anincrease in a reception error rate, etc. In this case, the receiverswitches to an open-loop MIMO communication scheme such as MMSE, BLAST,MMSE+BLAST, and Maximum Likelihood Estimation to avoid the deteriorationof decoding characteristics by continuing the closed-loop type. Further,the receiver informs the transmitter that the receiver has switched tothe open-loop spatial multiplexing communication mode. Alternatively, inresponse to the information from the transmitter that the transmitterhas switched to the open-loop spatial multiplexing communication mode,the receiver also may switch to the open-loop spatial multiplexingcommunication mode.

Further, according to a second embodiment of the present invention,there is provided a computer program that is described in acomputer-readable form so as to perform processing for spatiallymultiplexing and transmitting a signal to a receiver having a pluralityof antennas on a computer system in a transmitter having a plurality ofantennas. The computer program includes the steps of: performingclosed-loop spatial multiplexing communication based on feedbackinformation from the receiver, the closed-loop spatial multiplexingcommunication allowing the transmitter to perform weighted transmission;switching to open-loop spatial multiplexing communication in response tothe information that the amount of data to be transmitted at a time hasexceeded a predetermined amount of bits or a predetermined transmissiontime, the open-loop spatial multiplexing communication allowing thetransmitter to perform non-weighted transmission; and performing theopen-loop spatial multiplexing communication.

Furthermore, according to a third embodiment of the present invention,there is provided a computer program that is described in acomputer-readable form so as to perform processing for receiving amultiplexed signal from a transmitter having a plurality of antennas ona computer system in a receiver having a plurality of antennas. Thecomputer program includes the steps of: performing closed-loop spatialmultiplexing communication on the assumption that a transmission weighthas been applied by the transmitter, the closed-loop spatialmultiplexing communication allowing the receiver to perform weightedreception; switching to open-loop spatial multiplexing communication inresponse to a change in a reception characteristic, the open-loopspatial multiplexing communication allowing the receiver to performweighted reception; and performing the open-loop spatial multiplexingcommunication on the assumption that a transmission weight has beenapplied by the transmitter.

The computer program according to the second and third embodiments ofthe present invention is defined as a computer program described in acomputer-readable form so as to implement specified processes on acomputer system. In other words, when the computer program according tothe second and third embodiments of the present invention is installedin a computer system, the computer system exhibits cooperative effectsand operates as a communication apparatus. A plurality of suchcommunication apparatuses can be activated to construct a wirelessnetwork. In this manner, it is possible to provide effects similar tothose of the wireless communication system according to the firstembodiment of the present invention.

According to the embodiment of the present invention, there can beprovide a wireless communication system excellent at being able toexpand transmission capacity by communication in which spatialmultiplexing is utilized (MIMO communication), pairing a transmitterhaving a plurality of antennas with a receiver having a plurality ofantennas.

According to the embodiment of the present invention, there can beprovide a wireless communication system excellent at being able totransmit a large amount of data in series at a time with a MIMOcommunication scheme, avoiding the deterioration of decodingcharacteristics due to change over time by a channel matrix.

These and other features and advantages of the invention may be readilyascertained by referring to the following description of the embodimentand appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows Eb/No vs. bit error rate characteristics regarding eachcommunication scheme on the assumption of MIMO having three respectiveantennas for transmission and reception.

FIG. 2 schematically shows the function configuration of a transmitterwhich operates in a MIMO communication system in which a closed-looptype and an open-loop type coexist, according to an embodiment of thepresent invention.

FIG. 3 is a flowchart showing an operation process of a transmitter.

FIG. 4 shows an internal configuration of a transmission antennaweighting unit.

FIG. 5 schematically shows the function configuration of a receiverwhich operates in a MIMO communication system in which a closed-looptype and an open-loop type coexist, according to an embodiment of thepresent invention.

FIG. 6 is a flowchart showing an operation process of a receiver.

FIG. 7 schematically shows the configuration of a MIMOtransmitter/receiver of an open-loop type (example in related art).

FIG. 8 schematically shows the configuration of a MIMOtransmitter/receiver of a closed-loop type (example in related art).

FIG. 9 shows a configuration example of transmission data.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a MIMO communication system in which atransmitter having a plurality of antennas and a receiver having aplurality of antennas form a pair and spatially multiplex andcommunicate signals. In the description below, assume that the system isused in low-mobility environments such as in houses. First, adescription will be given to a closed-loop type of MIMO communicationscheme and its problem in the related art, and then a detaileddescription will be made of transmission/reception schemes according toan embodiment of the present invention with reference to the drawingsand equations.

First, a reception signal y is expressed as follows:y=Hx+n  (1)

where x is a transmission signal, H is propagation channel information,and n is a noise term.

By performing singular value decomposition (SVD) of the channel matrixH, the channel matrix H is expressed as follows:H=UDV^(H)   (2)

where a matrix V^(H) denotes a complex conjugate transpose matrix(Hamiltonian) of a matrix V. In general, the product of an originalmatrix multiplied by its complex conjugate transpose matrix is anidentity matrix (V·V^(H)=I) Therefore, in the closed-loop type of MIMOcommunication as shown in FIG. 8 for example, assuming that the receiveracquires the channel matrix H from TxPreamble and the transmitteracquires a transmission antenna weight V based on the feedbackRxPreamble from the receiver to perform weighted transmission using thetransmission antenna weight V, a reception signal y′ at the receiver isexpressed as follows:y′=H(Vx)  (3)=UDx

Further, assuming that the receiver performs singular valuedecomposition of the channel matrix H acquired from TxPreamble andperforms weighted reception using the obtained reception weight U^(H), areception signal y″ is expressed as follows:y″=U^(H)y′  (4)=U^(H)UDx=Dx

where D denotes a diagonal matrix and is expressed as follows:

$\begin{matrix}{D = \left\lbrack \begin{matrix}\sqrt{\lambda_{1}} & \; & \; \\\; & \sqrt{\lambda_{2}} & \; \\\vdots & \; & \ddots \\\; & \; & \; \\0 & \; & \;\end{matrix} \middle| \begin{matrix}\; & 0 \\\; & \; \\\; & \; \\\sqrt{\lambda_{n - 1}} & \; \\\; & \sqrt{\lambda_{n}}\end{matrix} \right\rbrack} & (5)\end{matrix}$

That is, in the SVD-MIMO transmission, UDV^(H) is obtained by performingthe singular value decomposition of a numerical matrix whose elementsdenote channel information corresponding to each antenna pair, namelythe channel information matrix H, and the transmission antenna weightingfactor matrix V and the reception antenna weighting factor matrix U^(H)are obtained. Thus, each MIMO channel is expressed as the diagonalmatrix D having the diagonal elements that are the square root of eacheigenvalue λ_(i), and signals can be multiplexed to be transmittedwithout any crosstalk.

Equation (5) shows that a transmission signal is given by power gain.Therefore, reception architecture in the closed-loop type of MIMOcommunication is basically the same as Zero Forcing (or Nulling) exceptfor the difference that H- or U^(H) is employed as the reception weight.

In such a case of transmitting a large amount of data in series at atime, there is a problem of change over time by the channel matrix. Thatis, in practice, the accuracy of the channel matrix H obtained from thepreamble information located at the beginning of a data division fromthe transmitter becomes deteriorated in the case of a large channelfluctuation or sending long data at a time. Assume that the receptionsignal y′ is originally obtained by the following equation:y′=H(Vx)  (6)=UDV^(H)(Vx)

However, assuming that the channel matrix changes to H_(new) due to apropagation path fluctuation, a practical reception signal y_(new) isexpressed as follows:y_(new)=H_(new)(Vx)  (7)=UnewDnewV^(H)new(Vx)

As a matter of course, even though the receiver receives signals using areception antenna weighting factor U_(new) based on H_(new) obtainedfrom the preamble information, orthogonal channels are not formed,crosstalk occurs between MIMO channels which should be logicallyindependent, and it results in a reception series in which interferencewaves remain.

This problem occurs because the accuracy of the transmission antennaweight V becomes deteriorated due to a fading channel fluctuation. Inthis embodiment, by switching from a closed-loop type of MIMOcommunication to an open-loop type of MIMO communication seamlessly, theaccuracy deterioration of the transmission weight V is coped with byoperation of the receiver alone to improve decoding characteristics.

The receiver stores an old V before measuring the accuracy of thetransmission weight matrix V. The receiver calculates a correlationvalue with a certain period, based on a new transmission weight matrixV_(new) estimated using a new transmission preamble (located at thebeginning of a data division), and compares the correlation value with areference threshold value. For example, a correlation value p can becalculated according to the following equation:

$\begin{matrix}{\rho = \frac{E\left\lbrack {V_{ij}^{*}V_{ij}^{new}} \right\rbrack}{\sqrt{E\left\lbrack {V_{ij}}^{2} \right\rbrack}\sqrt{E\left\lbrack {V_{ij}^{new}}^{2} \right\rbrack}}} & (8)\end{matrix}$

where E[•] denotes an ensemble average.

In this case, if a transmission preamble is multiplied by an oldtransmission weight matrix V, a new transmission weight matrix V_(new)can be obtained by multiplying H_(new)V (estimated from the preambleinformation) by V^(H) and calculating the following equation:H_(new)VV^(H)=H_(new)   (9)=UnewDnewV^(H)new

The accuracy deterioration of the transmission weight V is due to achannel matrix fluctuation. The accuracy deterioration of V may bedetected based on the deterioration of a reception error rate instead ofthe correlation between the old V and the new V_(new) by the accuracydeterioration of V.

When the receiver confirms that the accuracy of V has deterioratedbeyond the reference threshold value, the receiver stops performing theclosed-loop type of MIMO communication and determines an optimumreception scheme by the open-loop type of MIMO communication.

In this context, the optimum reception scheme refers to schemes such asMMSE (Minimum Mean Square Error), BLAST (Bell Layered Architecture SpaceTime coding), MMSE+BLAST, Maximum Likelihood Estimation, and the like.However, in the case where hardware design does not allow a plurality ofreception schemes, a reception scheme other than ZF is set to be readyfor use. FIG. 1 shows, for reference, Eb/No vs. bit error ratecharacteristics on the assumption of MIMO having three respectiveantennas for transmission and reception. As shown in FIG. 1, in general,better decoding characteristics of the open-loop type are obtained inthe following order. (It is known that the characteristic of MMSE aloneis nearly similar to that of BLAST.)

ZF<MMSE<BLAST<MMSE+BLAST<Maximum Likelihood Estimation

According to an algorithm selected above, the following equation isobtained based on the reception signal y_(new)=H_(new)(Vx) and using(H_(new)·V)⁻:(H_(new)V)⁻y_(new)=(H_(new)V)⁻·H_(new)(Vx)  (10)=x

In the case of also transmitting the preamble that is not multiplied bythe transmission weight matrix V, the preamble is decoded using thestored old V to make (H_(new)·V)⁻.

If receiving operation is performed using equation (10), at the time ofsignificant accuracy deterioration of V, it is anticipated that thedecoding characteristics are approaching those of ZF stochasticallyunder a complex iid. channel environment if the channel matrix H isupdated using the periodical preamble information inserted into datadivisions as shown in FIG. 9. That is, as a result of having broken offlearning of the channel matrix during long data communication, thedecoding characteristics of the receiver in the closed-loop type of MIMOcommunication become deteriorated to the same degree as those of ZFwhich has especially low decoding characteristics among MIMO schemes ofthe open-loop type.

In this embodiment, by switching reception schemes based on thefluctuation of the transmission weight V as described above, receptioncharacteristics of extremely higher quality can be maintained comparedto the case where only the closed-loop type continues to be used.Further, since signals are received using the closed-loop type at thebeginning, better reception characteristics are obtained compared to thecase where only the open-loop type is used. Furthermore, as a result, itis also possible to use only the open-loop type of reception scheme. Theopen-loop type of terminal can merely receive signals using (H_(new)·V)⁻calculated from the preamble, and the closed-loop type and the open-looptype of MIMO communication schemes can coexist.

Moreover, there may be methods for stopping the transmission in whichthe transmission antenna weight V is employed and switching to whollythe closed-loop type of MIMO scheme with the following methods. A firstmethod is as follows. The receiver detects the accuracy deterioration ofV and notifies the transmitter to perform the open-loop type ofcommunication with another communication device such as a PHS or asensor network device, so that multiplication of the transmissionantenna weight V is stopped. At this time, if different encoding ormodulation depth is applied to each transmission branch, it is necessaryto switch the encoding or the modulation depth as necessary beforetransmission.

A second method is as follows. After a series of procedures for theclosed-loop type is executed, the number of transmissions of continuousbits, a period, etc. by which the accuracy of the transmission antennaweight V can be maintained within a permissible limit are determinedbeforehand, and the transmitter and the receiver are switched to theopen-loop type of MIMO scheme based on the items predetermined above. Atthis time, if different encoding or modulation depth is applied to eachtransmission branch, it is necessary to switch the encoding or themodulation depth as necessary before transmission.

FIG. 2 schematically shows the function configuration of a transmitter10 which operates in a MIMO communication system in which theclosed-loop type and the open-loop type coexist, according to thisembodiment.

As shown in FIG. 2, the transmitter 10 has a plurality of transmissionantennas 11-1, 11-2, . . . , 11-m and performs the MIMO communicationusing a plurality of streams formed between the transmitter and areceiver (described later) having a plurality of reception antennas.

Data generation units 13-1, 13-2, . . . , 13-m encode and modulatetransmission data of each stream. A transmission antenna weighting unit12 applies an antenna weight to each stream in accordance with anoperational mode of either the closed-loop or the open-loop.

A control unit 14 controls transmission weighting in accordance with anoperational mode of either the closed-loop or the open-loop.

The transmitter 10 operates basically in the closed-loop type of MIMOcommunication mode, acquires an appropriate transmission antenna weightV based on feedback information from the receiver, and transmitsinformation more appropriately by adding the transmission weight.

In response to the information that the amount of data to be transmittedat a time has exceeded a predetermined amount of bits or a predeterminedtransmission time, the control unit 14 estimates that the channel matrixhas changed over time and switches to the open-loop type of MIMOcommunication mode. FIG. 3 shows an operation process of the transmitter10. This kind of operation prevents the decoding characteristics of thereceiver in the closed-loop type of MIMO communication from becomingdeteriorated to the same degree as those of ZF which has especially lowdecoding characteristics among MIMO schemes of the open-loop type.

The control unit 14 informs the receiver that the transmitter 10 sidehas switched to the open-loop type of MIMO communication mode.Alternatively, in response to the information from the receiver that thereceiver has switched to the open-loop type of MIMO communication mode,the control unit 14 may switch to the open-loop type of MIMOcommunication mode.

FIG. 4 shows an internal configuration of the transmission antennaweighting unit 12. As shown in FIG. 4, each stream transmission data ismultiplied by an antenna weight under the closed-loop type of MIMOcommunication mode. However, under the open-loop type of MIMOcommunication mode, the transmission antenna weighting unit 12 isswitched in such a way that data passes by, and the data is transmittedwithout a transmission weight.

FIG. 5 schematically shows the function configuration of a receiver 20which operates in a MIMO communication system in which the closed-looptype and the open-loop type coexist, according to this embodiment.

As shown in FIG. 5, the receiver 20 has a plurality of receptionantennas 21-1, 21-2, . . . , 21-n and performs the MIMO communicationusing a plurality of streams formed between the receiver and thetransmitter having a plurality of transmission antennas.

Data reproduction units 23-1, 23-2, . . . , 23-n demodulate and decodethe transmission data of each stream. A reception antenna weighting unit22 applies an antenna weight to each reception stream in accordance withan operational mode of either the closed-loop or the open-loop.

The receiver 20 operates basically in the closed-loop type of MIMOcommunication mode, acquires the channel matrix H based on a referencesignal from the transmitter, and transmits feedback information to thetransmitter. The receiver 20 transmits information more appropriately byadding the reception antenna weight.

According to a change in the transmission weight V obtained successivelyby singular value decomposition of the channel matrix and deteriorationof reception characteristics such as an increase in a reception errorrate, etc., a control unit 24 estimates that the channel matrix haschanged over time and switches to the open-loop type of MIMOcommunication mode. FIG. 6 shows an operation process of the receiver20. This kind of operation prevents the decoding characteristics in theclosed-loop type of MIMO communication from becoming deteriorated to thesame degree as those of ZF which has especially low decodingcharacteristics among MIMO schemes of the open-loop type.

The control unit 24 informs the transmitter 10 that the receiver 20 sidehas switched to the open-loop type of MIMO communication mode.Alternatively, in response to the information from the transmitter 10that the transmitter has switched to the open-loop type of MIMOcommunication mode, the control unit 24 may switch to the open-loop typeof MIMO communication mode. The information that the receiver side hasswitched to the open-loop type of spatial multiplexing communicationmode may be sent to the transmitter by the MIMO communication which isthe same as the data communication channel; however, the information maybe sent using a wireless communication scheme other than the MIMOcommunication channel, such as a wireless LAN transmission channel, asensor network, or the like.

The reception antenna weighting unit 22 determines an antenna weightaccording to a reception scheme determined by the control unit 24. Underthe open-loop type of MIMO communication mode, there are appliedcommunication schemes other than Zero-forcing, such as MMSE, BLAST,MMSE+BLAST, Maximum Likelihood Estimation, etc.

In each reception architecture of the open-loop and the closed-loop,circuit processing units including generalized inverse matrix arithmeticcircuits such as Moore Penrose and LU decomposition, a complex conjugatemultiplication circuit for antenna weighting, and arithmetic circuitsregarding CDM and OFDM decoding can be shared among reception schemes.

The present invention has been described in detail with reference to aspecific embodiment. However; it is obvious that those skilled in theart can alter or modify the embodiment without departing from the scopeand sprit of the invention. That is, the present invention has beendisclosed in the form of exemplification, and the contents of thespecification should not be interpreted restrictively. To understand thesubject matter of the present invention, the appended claims should betaken into consideration.

What is claimed is:
 1. A wireless communication system in which atransmitter having a plurality of antennas and a receiver having aplurality of antennas form a pair and spatially multiplex andcommunicate a signal, the wireless communication system comprising: aclosed-loop spatial multiplexing communication mode in which thetransmitter transmits a signal for acquiring channel information to thereceiver and the receiver sends feedback of the channel information tothe transmitter; an open-loop spatial multiplexing communication mode inwhich only the transmitter transmits a signal for acquiring channelinformation to the receiver; and a mode switcher configured to switchbetween the closed-loop spatial multiplexing communication mode and theopen-loop spatial multiplexing communication mode, and switch to theopen-loop spatial multiplexing communication mode in response toinformation indicating that an amount of data to be transmitted hasexceeded a predetermined data limit or a transmission time during datatransmission under the closed-loop spatial multiplexing communicationmode has exceeded a predetermined time limit.
 2. The wirelesscommunication system of claim 1, wherein the mode switcher is furtherconfigured to switch to the open-loop spatial multiplexing communicationmode in response to the information that an error rate has exceeded apredetermined value during data transmission under the closed-loopspatial multiplexing communication mode.
 3. The wireless communicationsystem according to claim 1, wherein the transmitter transmits data thatthe transmitter does not apply a transmission weight to under theopen-loop spatial multiplexing communication mode.
 4. The wirelesscommunication system of claim 1, wherein the receiver performs anopen-loop spatial multiplexing communication operation other than acommunication scheme for providing a reception weight based on aninverse matrix H⁻ of a channel matrix obtained from a propagation pathcondition under the open-loop spatial multiplexing communication mode.5. The wireless communication system according to claim 1, wherein saidmode switcher is further configured to switch between the closed-loopspatial multiplexing communication mode and the open-loop spatialmultiplexing communication mode, and inform the receiver that a mode hasbeen switched to the open-loop spatial multiplexing communication mode.6. A wireless communication system in which a transmitter having aplurality of antennas and a receiver having a plurality of antennas forma pair and spatially multiplex and communicate a signal, the wirelesscommunication system comprising: a closed-loop spatial multiplexingcommunication mode in which the transmitter transmits a signal foracquiring channel information to the receiver, the receiver sendsfeedback of the channel information to the transmitter, the receiverfinds a channel matrix H by receiving a reference signal sent from thetransmitter, obtains a reception weight matrix U^(H), a diagonal matrixD and a transmission weight matrix V by performing singular valuedecomposition of the channel matrix H into UDV^(H), and sends feedbackof the transmission weight matrix V to the transmitter, there isperformed Singular Value Decomposition-Multiple Input Multiple Output(SVD-MIMO) communication in which the transmitter performs weightedtransmission with the transmission weight matrix V and the receiverperforms weighted reception with the reception weight matrix U^(H); anopen-loop spatial multiplexing communication mode in which only thetransmitter transmits a signal for acquiring channel information to thereceiver; and a mode switcher configured to switch between theclosed-loop spatial multiplexing communication mode and the open-loopspatial multiplexing communication mode, and switch to the open-loopspatial multiplexing communication mode in response to the informationthat the transmission weight matrix V obtained by the singular valuedecomposition has changed beyond a predetermined value during datatransmission under the closed-loop spatial multiplexing communicationmode.
 7. A wireless communication apparatus having a plurality ofantennas that spatially multiplexes and transmits a signal to a receiverhaving a plurality of antennas, the wireless communication apparatuscomprising: a closed-loop spatial multiplexing communication mode inwhich the apparatus performs weighted transmission based on feedbackinformation from the receiver; an open-loop spatial multiplexingcommunication mode in which the apparatus performs non-weightedtransmission; and a mode switcher configured to switch between theclosed-loop spatial multiplexing communication mode and the open-loopspatial multiplexing communication mode, and switch to the open-loopspatial multiplexing communication mode in response to informationindicating that an amount of data to be transmitted has exceeded apredetermined data limit or a transmission time during data transmissionunder the closed-loop spatial multiplexing communication mode hasexceeded a predetermined time limit.
 8. The wireless communicationapparatus of claim 7, wherein said mode switcher is further configuredto switch between the closed-loop spatial multiplexing communicationmode and the open-loop spatial multiplexing communication mode, andswitch to the open-loop spatial multiplexing communication mode inresponse to information that an error rate has exceeded a predeterminedvalue during data transmission under the closed-loop spatialmultiplexing communication mode.
 9. A wireless communication apparatushaving a plurality of antennas that spatially multiplexes and transmitsa signal to a receiver having a plurality of antennas, the wirelesscommunication apparatus comprising: a closed-loop spatial multiplexingcommunication mode in which the apparatus performs weighted transmissionbased on feedback information from the receiver; an open-loop spatialmultiplexing communication mode in which the apparatus performsnon-weighted transmission; and a mode switcher configured to switchbetween the closed-loop spatial multiplexing communication mode and theopen-loop spatial multiplexing communication mode, and switch to theopen-loop spatial multiplexing communication mode in response to theinformation from the receiver that a mode has been switched to theopen-loop spatial multiplexing communication mode.
 10. A wirelesscommunication apparatus having a plurality of antennas that spatiallymultiplexes and transmits a signal to a receiver having a plurality ofantennas, the wireless communication apparatus comprising: a closed-loopspatial multiplexing communication mode in which the apparatus performsweighted transmission based on feedback information from the receiver;an open-loop spatial multiplexing communication mode in which theapparatus performs non-weighted transmission; a mode switcher configuredto switch between the closed-loop spatial multiplexing communicationmode and the open-loop spatial multiplexing communication mode; afeedback information receiving section configured to receive feedbackinformation for acquiring a transmission weight matrix V from thereceiver and perform Singular Value Decomposition-Multiple InputMultiple Output (SVD-MIMO) transmission of weighted transmission by thetransmission weight matrix V under the closed-loop spatial multiplexingcommunication mode; and a transmitting section configured to transmitdata to which the apparatus does not apply the transmission weight Vunder the open-loop spatial multiplexing communication mode.
 11. Awireless communication apparatus having a plurality of antennas thatreceives a multiplexed signal from a transmitter having a plurality ofantennas, the wireless communication apparatus comprising: a closed-loopspatial multiplexing communication mode in which the apparatus sendsfeedback of channel information to the transmitter, finds a channelmatrix H by receiving a reference signal sent from the transmitter,obtains a reception weight matrix U^(H), a diagonal matrix D and atransmission weight matrix V by performing singular value decompositionof the channel matrix H into UDV^(H), and performs Singular ValueDecomposition-Multiple Input Multiple Output (SVD-MIMO) communicationusing weighted reception with the reception weight matrix U^(H) andsends feedback of the transmission weight matrix V to the transmitter;an open-loop spatial multiplexing communication mode in which theapparatus does not send feedback of channel information to thetransmitter; and a mode switcher configured to switch between theclosed-loop spatial multiplexing communication mode and the open-loopspatial multiplexing communication mode, and switch to the open-loopspatial multiplexing communication mode in response to the informationthat the transmission weight matrix V obtained by the singular valuedecomposition has changed beyond a predetermined value during datatransmission under the closed-loop spatial multiplexing communicationmode.
 12. A wireless communication apparatus having a plurality ofantennas that receives a multiplexed signal from a transmitter having aplurality of antennas, the wireless communication apparatus comprising:a closed-loop spatial multiplexing communication mode in which theapparatus sends feedback of channel information to the transmitter; anopen-loop spatial multiplexing communication mode in which the apparatusdoes not send feedback of channel information to the transmitter; and amode switcher configured to switch between the closed-loop spatialmultiplexing communication mode and the open-loop spatial multiplexingcommunication mode, and inform the transmitter that a mode has beenswitched to the open-loop spatial multiplexing communication mode. 13.The wireless communication apparatus of claim 12, wherein the modeswitcher is further configured to switch to the open-loop spatialmultiplexing communication mode in response to the information from thetransmitter that a mode has been switched to the open-loop spatialmultiplexing communication mode.
 14. A wireless communication apparatushaving a plurality of antennas that receives a multiplexed signal from atransmitter having a plurality of antennas, the wireless communicationapparatus comprising: a closed-loop spatial multiplexing communicationmode in which the apparatus sends feedback of channel information to thetransmitter; an open-loop spatial multiplexing communication mode inwhich the apparatus does not send feedback of channel information to thetransmitter; a mode switcher configured to switch between theclosed-loop spatial multiplexing communication mode and the open-loopspatial multiplexing communication mode; and a communication sectionconfigured to perform an open-loop spatial multiplexing communicationoperation other than a communication scheme for providing a receptionweight based on an inverse matrix I-V of a channel matrix obtained froma propagation path condition under the open-loop spatial multiplexingcommunication mode.
 15. A wireless communication method for spatiallymultiplexing and transmitting a signal to a receiver having a pluralityof antennas in a transmitter having a plurality of antennas, thewireless communication method comprising: performing closed-loop spatialmultiplexing communication based on feedback information from thereceiver, the closed-loop spatial multiplexing communication allowingthe transmitter to perform weighted transmission; switching to open-loopspatial multiplexing communication in response to information indicatingthat an amount of data to be transmitted has exceeded a predetermineddata limit or a transmission time during data transmission under theclosed-loop spatial multiplexing communication mode has exceeded apredetermined time limit; and performing the open-loop spatialmultiplexing communication.
 16. A wireless communication method forreceiving a multiplexed signal from a transmitter having a plurality ofantennas in a receiver having a plurality of antennas, the wirelesscommunication method comprising the steps of: performing closed-loopspatial multiplexing communication on the assumption that a transmissionweight has been applied by the transmitter, the closed-loop spatialmultiplexing communication allowing the receiver to perform weightedreception; switching to open-loop spatial multiplexing communication inresponse to a change in a reception characteristic, the open-loopspatial multiplexing communication allowing the receiver to performweighted reception; performing the open-loop spatial multiplexingcommunication on the assumption that a transmission weight has beenapplied by the transmitter.
 17. A computer-readable medium storing acomputer program which when executed on a computer causes the computerto perform processing for spatially multiplexing and transmitting asignal to a receiver having a plurality of antennas from a transmitterhaving a plurality of antennas, comprising: performing closed-loopspatial multiplexing communication based on feedback information fromthe receiver, the closed-loop spatial multiplexing communicationallowing the transmitter to perform weighted transmission; switching toopen-loop spatial multiplexing communication in response to informationindicating that an amount of data to be transmitted has exceeded apredetermined data limit or a transmission time during data transmissionunder the closed-loop spatial multiplexing communication mode hasexceeded a predetermined time limit; and performing the open-loopspatial multiplexing communication.
 18. A computer-readable mediumstoring a computer program which when executed on a computer causes thecomputer to perform processing for spatially multiplexing andtransmitting a signal to a receiver having a plurality of antennas, in atransmitter having a plurality of antennas, comprising: performingclosed-loop spatial multiplexing communication by performing weightedtransmission based on feedback information from the receiver; performingopen-loop spatial multiplexing communication in which the transmitterperforms non-weighted transmission; switching between the closed-loopspatial multiplexing communication and the open-loop spatialmultiplexing communication, and switching to the open-loop spatialmultiplexing communication in response to information from the receiverthat a mode has been switched to the open-loop spatial multiplexingcommunication mode.
 19. An information processing apparatus used for acommunication system including a transmitter having a plurality ofantennas and a receiver having one or more antennas, comprising:circuitry configured to: perform wireless communication in a closed loopspatial multiplexing communication mode in which feedback informationfor weighting signals is received from the receiver or a differentspatial coding communication mode in which the feedback information isnot received; and switch to the different spatial coding communicationmode from the closed loop spatial multiplexing communication mode inresponse to an indication from the receiver according to time expiry indata transmission, wherein the feedback information is transmitted via aMIMO (Multiple-Input Multiple-Output) communication channel and theindication is notified via another communication channel that isdifferent from the MIMO communication channel.
 20. A method, implementedby an information processing apparatus used for a communication systemincluding a transmitter having a plurality of antennas and a receiverhaving one or more antennas, the method comprising: performing wirelesscommunication in a closed loop spatial multiplexing communication modein which feedback information for weighting signals is received from thereceiver or a different spatial coding communication mode in which thefeedback information is not received; and switching to the differentspatial coding communication mode from the closed loop spatialmultiplexing communication mode in response to an indication from thereceiver according to time expiry in data transmission, wherein thefeedback information is transmitted via a MIMO (Multiple-InputMultiple-Output) communication channel and the indication is notifiedvia another communication channel that is different from the MIMOcommunication channel.
 21. A receiver for a communication system thatincludes a transmitter having a plurality of antennas and the receiverhaving one or more antennas, the receiver comprising: circuitryconfigured to: perform wireless communication in a closed loop spatialmultiplexing communication mode in which feedback information forweighting signals is transmitted to the transmitter or a differentspatial coding communication mode in which the feedback information isnot transmitted; and cause the transmitter to switch to the differentspatial coding communication mode from the closed loop spatialmultiplexing communication mode in response to an indication from thereceiver according to time expiry in data transmission, wherein thefeedback information is transmitted via a MIMO (Multiple-InputMultiple-Output) communication channel and the indication is notifiedvia another communication channel that is different from the MIMOcommunication channel.